The present invention relates to a method and means for forming dense articles and articles of irregular configuration by plasma deposition. More particularly it relates to a low pressure plasma deposition process and apparatus by which dense cohesive deposits which have intricate shapes are formed on larger size receiving surfaces. By larger size as the term is used herein is meant a size substantially larger than the area of a receiving surface which is coated with dense deposit as a single stationary plasma gun applies a low pressure plasma deposited layer onto a stationary receiving surface.
The state of the art of low pressure plasma deposition makes possible the deposit of a dense layer in the central portion of the target area within the sweep of a plasma flame. For a particular apparatus and set of operating parameters this central region will be approximately 20 to 40 sq. cm. in diameter and the deposit densities approach about 100% particularly if the deposited layer is given a densification heat treatment. Also typically the spray deposit surrounding the central region, and particularly in a fringe region, is less dense and in fact becomes extremely porous outside an area of about 100 sq. cm. The porous outer zone is not densified to even 97% of theoretical density and material with density of less than 97% has poor combinations of physical properties, and in particular poor tensile properties.
One reason why a deposit at its outer fringes is less dense and in other respects has less desirable properties is that the angle of incidence of the deposit from the gun is not at right angles or at 90.degree.. It has been found that deposit from a plasma flame which is incident on a receiving surface at an acute angle substantially different from 90.degree. has poorer properties. Also the properties deteriorate more the more the angle is different from 90.degree..
To put this in perspective and using circular areas a designated central area of dense deposit of 20 square centimeters covers an area having a diameter of about 5 centimeters. If only the central area is dense as deposited then only a small fraction of the whole deposit is dense. 40 square centimeters is included within a circle having a diameter of about 7.1 centimeters and the 100 square centimeter area is included within a circle having a diameter of about 11.3 centimeters.
Under present technology if the size of the deposit to be made from a plasma gun is larger in at least one dimension than the dense region of a spray pattern, then it is necessary to use either a gun motion or substrate motion, or both, to cover the larger area. This motion leads to a deposit that is some combination of dense and porous. The effect of increasing the deposit size on the tensile and ductility properties of the deposit leads to the conclusion that larger area deposits are less dense and are weaker in the as-deposited state.
Also, in general where the deposition angle (meaning the acute angle between the direction of the spray and the surface on which the spray is deposited) is low then the density and tensile properties of the deposit are further reduced. For example, if the deposition angle is less than 70.degree. this leads to a further reduction in density and tensile properties of the deposit over those found for the layers deposited with the gun aimed normal to the receiving surface.
Where the receiving surface itself is non-planar, and particularly when the surface has a complex geometry, these parts of the surface which are not aimed normal to the plasma gun will receive the plasma spray at angles other than the desirable 90.degree. which leads to the high density deposit.
Plasma spray deposits have been formed from numerous powdered starting materials including powders of nickel base superalloys.
It has been found that the ductility values of deposits which have less than a 97% density after heat treatment, as, for example, at about 1250.degree. for nickel base superalloys for a suitable time, is low.